The present invention relates in general to a process and apparatus for quantum imaging using entangled photon pairs, and in particular to the properties of quantum imaging used to generate an image of an object via correlations between photons or quantum particles reflected or absorbed by the object and with photons or quantum particles that have never interacted.
By way of background, the non-classical two-photon interaction or quantum entanglement was described by Albert Einstein et al. (Einstein, Podolsky, Rosen paradox), “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” Physical Review, Volume 47, May 15, 1935, pgs. 777-800. The paradox of quantum entanglement, as described therein, relates to the concept that as a result of the process of measurement of a first system, using quantum mechanics, two different physical quantities are obtainable in the second system, despite the fact that at the time of the measurements, the two systems no longer interact and the second system is not disturbed in any way by the first. Einstein, et al, were unable to reconcile this quantum mechanical description of reality with the so-called classical physics determination that no “real” change can take place in the second system as a consequence of anything that may be done to the first system after the two systems no longer interact.
Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more quantum particles are linked together such that the quantum state of one quantum particle appears to interact with its counterpart; even though the individual quantum particles may be spatially separated. This apparent interconnection leads to correlations between observable physical properties of remote systems, since the interaction of the remote system with quantum state of one of a pair can be observed though observation of the counterpart. For example, according to quantum mechanics, the spin of a quantum particle is indeterminate until such time as some physical intervention is made to measure the spin; which, in general, could equally be spin-up or spin-down. However, when two members of an entangled pair are measured, one will always be spin-up and the other will be spin-down, regardless of the distance between the two particles. It is normally taught in quantum theory that no hidden variable theory can account for these results of quantum mechanics. The statistics of multiple measurements must generally relate to an inequality (called Bell's inequality), which is violated both by quantum mechanical theory and experimental results.
In U.S. application Ser. No. 12/330,401, hereby incorporated by reference, there is a general discussion of the need for the ability to image through obscuring media such as smoke or clouds, which is a problem faced by satellite imaging analysts, firefighters, drivers, oceanographers, astronomers, military personnel, and medical personnel. Conventional imaging techniques have, to a large extent, arrived at the theoretical limits of image resolution owing to wavelength-limited resolution, optical element distortions, and the reflective interaction between photons and an object to be imaged.
For ease of understanding, the terminology “test path” will be used to designate the path or beam of the photon(s) entering the object or target area. The terminology “reference path” will be used to designate the beam or path that the reference photon(s) travels.
As stated in U.S. application Ser. No. 12/330,401, incorporated by reference, ghost imaging holds the prospect of improving image resolution. Currently, quantum ghost imaging is largely dependent on the transmission of electromagnetic waves (photons) through the object to be imaged.
The theoretical work reported by Klyshko in “Combined EPR and Two-Slit Experiments: Interference of Advanced Waves”, Physics Letters A, Volume 132, number 6.7, pp. 299-304 (1988) see also, Sov. Phys. Usp. 31, 74 suggested a non-classical two-photon interaction could exist.
The first two-photon imaging experiment was reported by Pittman et al., in “Optical Imaging by Means of Two-photon Quantum Entanglement,” Physical Review, A, Vol. 52, No. 5, November 1995. According to the Pittman article, a two-photon optical imaging experiment was performed to test the two-particle entanglement as described by Albert Einstein et al. (Einstein, Podolsky, Rosen), referenced above, to determine if there was a correlation in position and in momentum for an entangled two-photon system; using “test beam or path” and “reference beam or path” photon pairs. Specifically, an aperture placed in front of a fixed detector was illuminated by a signal beam through a convex lens. A sharp magnified image of the aperture was found in the coincidence counting rate when a mobile detector was scanned in the transverse plane of the reference beam at a specific distance in relation to the lens. The experiment was named “ghost imaging” due to its surprising nonlocal feature.
Additional experiments are reported in Pittman, et al. “Optical Imaging by Means of Two-Photon Entanglement,” Phys. Rev. A, Rapid Comm., Vol. 52, R3429 (1995) and ghost interference by Strekalov, et al, “Observation of Two-Photon ‘Ghost’ Interference and Diffraction,” Phys. Rev. Lett., Vol. 74, 3600 (1995), which together stimulated the foundation of quantum imaging in terms of multi-photon geometrical and physical optics. The prior art transmissive ghost imaging optical scheme using entangled photons of Pittman et al. is depicted in FIG. 1.
Boto and colleagues (Boto, Agedi, et al.), in “Quantum Interferometric Optical Lithography: Exploiting Entanglement to Beat the Diffraction Limit”, Physical Review Letters, Volume 85, Number 13, 25 Sep. 2000, The American Physical Society, pgs. 2733-2736, developed an entangled multi-photon system for sub-diffraction-limited imaging lithography and proposed a heuristic multiphoton absorption rate of a “noon” state and proved that the entangled N-photon system may improve the spatial resolution of an imaging system by a factor of N, despite the Rayleigh diffraction limit. The working principle of quantum lithography was experimentally demonstrated by D′Angelo, Milena, et al., in “Two-Photon Diffraction and Quantum Lithography”, Physical Review Letters, Volume 87, Number 1, Jul. 2, 2001, pgs. 1-4, by taking advantage of an entangled two-photon state of spontaneous parametric down-conversion.
Quantum imaging has so far demonstrated two peculiar features: (1) reproducing ghost images in a “nonlocal” manner, and (2) enhancing the spatial resolution of imaging beyond the diffraction limit. Both the nonlocal behavior observed in the ghost imaging experiment and the apparent violation of the uncertainty principle explored in the quantum lithography experiment are due to the two-photon coherent effect of entangled states, which involves the superposition of two-photon amplitudes, a nonclassical entity corresponding to different yet indistinguishable alternative ways of triggering a joint-detection event in the quantum theory of photodetection as articulated by Glauber in “The Quantum Theory of Optical Coherence”, Physical Review, Volume 130, Number 6, pp. 2529-2539, Jun. 15, 1963, and “Coherent and Incoherent States of the Radiation Field”, Physical Review, Volume 131, Number 6, 15, pp. 2766-2788, September 1963. The nonlocal superposition of two-photon states may never be understood classically.
For further discussion, see U.S. application Ser. No. 12/330,401, hereby incorporated by reference. The ongoing lack of theoretical understanding of ghost imaging has hampered efforts to develop reflective ghost imaging systems for practical field uses in such fields as satellite, field, medical and research imaging. Moreover, there exists a need for a system using ghost image where feedback or measurement is not possible at the target area.